Modern electronic devices such as a notebook computer comprise a variety of memories to store information. Memory circuits include two major categories. One is volatile memories; the other is non-volatile memories. Volatile memories include random access memory (RAM), which can be further divided into two sub-categories, static random access memory (SRAM) and dynamic random access memory (DRAM). Both SRAM and DRAM are volatile because they will lose the information they store when they are not powered. On the other hand, non-volatile memories can keep data stored on them. Non-volatile memories include a variety of sub-categories, such as read-only-memory (ROM), electrically erasable programmable read-only memory (EEPROM) and flash memory.
One type of EEPROM memory device is referred to as a flash memory device. Flash memories have become increasingly popular in recent years. A typical flash memory comprises a memory array having a large number of memory cells arranged in rows and columns. Each of the memory cells is fabricated as a field-effect transistor having a drain region, a source region, a control gate and a floating gate.
The floating gate is disposed above a substrate. The floating gate is between the source region and the drain region, but separated from them by an oxide layer. The floating gate may be formed of suitable materials such as polycrystalline silicon (“poly”) and/or some other conductive materials. The oxide layer may be formed of silicon dioxide (SiO2) and/or the like. The control gate may be disposed over the floating gate. The control gate and the floating gate may be separated by a thin oxide layer.
In operation, a floating gate is capable of holding a charge and is separated from source and drain regions contained in a substrate by an oxide layer. Each of the memory cells may be electrically charged by injecting electrons from the substrate through the oxide layer. The charge may be removed from the floating gate by tunneling the electrons to the source region or an erase gate during an erase operation. The data in flash memory cells are thus determined by the presence or absence of electrical charges in the floating gates.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.